Antibody-drug conjugates are innovative drugs designed to combine the unique and highly specificy properties of monoclonal antibodies (mAbs) with cell killing properties of selected conjugated drugs. Key components of antibody-drug conjugates usually include a monoclonal antibody or similar molecule, a stable or metastable linker, and a cytotoxic cell killing drug. Antibody-drug conjugates are designed to target specific cancer or tumor tissue or cells.

Antibody-drug conjugates

(ADC) are composed of two moieties covalently linked:

  •  An antibody that usually is a monoclonal antibody (mAb) and that acts as the targeting component, i.e. it identifies and binds to a specific antigen on the surface of the targeted cell, usually a cancerous cell, or a tumor cell.
  •  A cytotoxic drug that after entering the targeted cells proceeds by various mechanisms to kill them. 
  •  In all cases, the conjugation method shown maintains the structural and functional characteristics of both the antibody and drug.
  • The antibody, usually a mAb, recognizes and to a specific epitope on certain antigen that usually is either found exclusively in some cells, e.g. virally infected cells or expressed several thousand times higher than in normal cells. 
  • Preservation of a mAb recognition site is critical for its effective binding to the target cell. Hence, conjugation should fill the following requirements:
  • The conjugated drug should be distant from the antigen binding site in the Fab region, i.e. it should be conjugated at the Fc region.
  • Conjugation can occur by using one of the two disulfide bonds at the Fc region or at the oligosaccharide that is present in the Fc region.
  • The drug should have available some functional groups, such as –COOH, -CHO, -SH, -NH2 and others, capable of being used in the conjugation process but without affecting the drug’s pharmacological activity. 
  • Because antibodies are made of two identical parts composed each of a Fab and Fc contiguous sectors, which are linked to each other via two disulfide bonds, usually one disulfide bond is reduced to product two sulfhydryl groups while keeping one disulfide intact to maintain the structural integrity of the antibody.
  • Hence, controlled reduction provides two –SH groups per antibody molecule, to which it is possible to attach one or two molecules of the drug.
  • Alternatively, the oligosaccharide on each Fc chain can be oxidize using periodic acid to open a sugar ring and deliver two aldehyde groups which can be reacted with an available amino group in the drug.
  • Upon binding of the mAb to the corresponding antigen on the cell surface, the ADC is internalized into the cell by endocytosis, where the drug will interact with its therapeutic target.
  • The capacity of the drug to interact with its therapeutic target means that the drug is free from the mAb and capable of binding to specific receptors in the cell.
  • To assure the release of the drug from the mAb once the ADC is inside the cell, the spacer linking the drug to the mAb should be cleaved once is inside the cell, a goal achieved by using cleavable spacers.
  • Cleavable spacers are those that can be cleave by an enzyme like a protease, reduced inside the cell by –SH group, or split by acid pH in the lysosomal compartment.
  • Because of the availability of potentially two reactive groups per mAb, there will usually be two drug molecules per mAb. 
  • If needed the amount of drug per mAb may be increased between limits, by using branched linkers.
  • Conjugates can be qualified 1st by SDS-PAGE with and without reduction of the disulfide bonds.
  • Further characterization can be made by MALDI and in some cases spectrophotometrically.